In the development and manufacture of integrated circuits, the capabilities of Very Large Scale Integrated (VLSI) and Ultra Large Scale Integrated (ULSI) circuits are driven by different fabrication technologies, but optical lithography is typically recognized as the most critical. Technology generations of integrated circuits are measured in terms of lithographic minimum feature sizes, e.g., sub-micron linewidths.
The ever-increasing demand for smaller circuit features and higher packing densities has mainly been satisfied by increasing the numerical aperture of a lens system in a microlithographic tool. However, the performance of most lithographic lenses is diffraction limited. The three variables that determine this limit are (1) the lens numerical aperture, (2) the wavelength of the exposing radiation, and (3) the coherence of the exposing radiation. Rayleigh criteria may be used as an approximation for determining the minimum resolution and the total focus depth of a diffraction-limited system. Regarding minimum resolution, the equation is as follows: EQU Minimum Resolution=k1 (exposing wavelength/NA)
where NA is the numerical aperture and k1 is a scaling factor which depends on the lens aberration and a number of other factors. Generally, it is assumed that k1=0.8 for the present generation of lithographic tools.
The depth of focus for the minimum feature resolved according to the equation above is determined as follows: EQU Depth of Focus=k2 (exposing wavelength/(NA).sup.2)
where k2 is a scaling factor that is dependent upon lens aberrations and processing factors. A nominal value of 0.8 is assumed for k2.
From the equations above, it can be seen that an increase in the numerical aperture of the projection optics achieves superior resolutions, but at the expense of the depth of focus. The range of acceptable focus is further reduced by such factors as the existing topography of the substrate, any non-flatness of the substrate, the precision of the lithographic tool's auto focus system, and the error associated with the process of determining the optimal focus.
It has recently been found that the optimal focus, as determined by the lithographic tool's auto focus system, is a function of the type of film on the surface of the substrate. For instance, as determined by the tool, there may be a 0.4 .mu.m offset of the best focus plane for a poly gate level as compared to a contact mask. It has also been determined that the optimal focus is a function of the existing topography on the substrate. This has been observed as a 0.2 .mu.m shift in the best focus plane between the poly gate level for a memory circuit and the same level for a CPU chip. These results indicate that accurate determination of optimal focus is a critical factor in achieving the maximum depth of focus that is available from a particular projection lens.
As defined herein, a "lithographic system" is defined as including components (1) for the transfer of an image through a projection system, (2) for the interaction of projected flux with a resist system, and (3) for the development of the image into the resist.
There are three conventional methods of determining how accurately a desired image is being reproduced upon passage through a lithographic system. The first method is described in U.S. Pat. No. 4,890,239 to Ausschnitt et al. This method relies on the measurement of the linewidth of a lithographic feature over a range of focus positions. A graph of linewidth versus focus is formed for the series of focal settings. The optimal focus is extracted by selecting the value at which the linewidth is least sensitive to exposure, i.e., where the curves of linewidth versus focus are most tightly bunched together. While this method has been acceptable in the past, it has severe shortcomings that are a direct result of the above-described trend of increasing the numerical aperture. For tools associated with a very high numerical aperture, curves of linewidth versus focus have become asymmetrical. Thus, deducing the optimal focus using this method has become more difficult.
A second method of choosing optimal focus is based on obtaining cross sections of a set of parallel lines or contacts in an SEM (Scanning Electronic Microscope) over a range of focus positions. Sidewall angle is then plotted as a function of focus, with sidewall angle being defined as the angle of a wall of photoresist relative to a wafer surface. Optimal focus is determined by selecting the maximum of the plot. Unlike the method described in Ausschnitt, this method works well even for cases in which curves are asymmetrical. However, obtaining cross sections of the lines or contacts requires destructive and dirty cleaving of a sample and is labor intensive.
A third method used in identifying an optimal focus is one which requires an operator to visually identify the best focus in an electron or optical microscope by inspecting the "quality" of the image as a function of focus. This method is operator-dependent and exhibits large operator-to-operator variations.
An object of the invention is to provide a method and apparatus for determining optimal settings for operation variants of a feature-forming system.